Observational Strategies toward Capturing Peak Wind Speeds in Landfalling Hurricanes

Following on from the 2017 hurricane season, the 2018 season has afforded us some important opportunities to observe high-impact land-falling hurricanes along vulnerable stretches of the United States coastline. In this paper, we describe the landfall surface wind speed observation datasets for Hurricanes Florence and Michael. Both were large and powerful storms as they made landfall, but in different phases of their evolution. Florence was losing intensity as it approached landfall and came ashore near Wilmington, NC at a crawl as a Category 1 hurricane. Conversely, Michael was fast moving and rapidly strengthening Category 4 hurricane before it made landfall near Mexico Beach, FL. We go on to discuss the lessons learned over the past two seasons, the strengths and weakness of the current surface wind speed observing systems, and describe a consolidated surface observation strategy that seeks to increase our ability to capture the strongest land-falling wind speeds and characterize their distribution. Reliable, real-time, ground-truth wind speed observations are critical to managing emergency response and recovery and understanding the future risk that these storms pose.

The surface observation datasets come from a wide variety of station types and providers and include data from the well-known federal networks, as well as state, local, academic, and commercial mesonets. Most of these in-situ assets are not hurricane hardened and many go down as peak winds arrive due to power or communications outages, as they are damaged by flying debris or structural failures, or as the instruments themselves fail. A smaller subset of these assets are hardened for landfall impacts, including WeatherFlow’s Hurricane Network and the deployable hurricane networks of the University of Florida (FCMP Towers) and Texas Tech University (StickNet). These hardened stations have a history of surviving direct hits, although lessons are still being leaned about making them even more robust. It is noteworthy that only a few hardened observational platforms survived the fury of Michael’s landfalling core.

The NHC’s storm intensity estimates are the maximum representative wind speed values associated with the cyclone’s circulation that are expected anywhere within the storm at the specified advisory or forecast time. The aerial coverage that is expected to see the maximum wind intensity is likely to be very small relative to the storm’s core size and, as such, the intensity estimates are not wholly representative of general conditions throughout the storm. Previously we reconciled the NHC intensity estimates and observed wind speeds for 2017 Hurricanes Harvey and Irma and demonstrated that, within the uncertainties, the NHC’s intensity estimates are consistent with the open ocean exposure wind speeds observed by reconnaissance aircraft as the storms approached landfall. However, inconsistencies quickly arise as the storm makes landfall and the surface wind field is rapidly impacted and complicated by the complex coastal orography. On land, even considering exposure and the uncertainties, the NHC intensity estimates tend to exceed what is observed by a Saffir Simpson category or more. We see these trends carried through into the 2018 hurricane season with Florence and Michael. The NHC intensity estimate definition is not well understood outside of the meteorological community and, as a result, leads to challenges in communicating risk. A more complete and representative dataset is necessary for the meteorological community to more fully understand the wind speed characteristics of land-falling hurricanes, which will enable it to better communicate hurricane risks to the wider community.